The measurement of the speed of vehicles is an on-going issue for ensuring road safety. Thus, studies have demonstrated that the number of road accidents is directly correlated with the average speed of the vehicles.
At the present time, several devices exist for measuring the speed of automobile vehicles:
For example, fixed or mobile radar devices are noted. These installations which allow a precise measurement of the speed to be made are very costly and complex.
Laser sighting devices use the Doppler effect for the speed measurement and also allow a precise measurement of the speed of vehicles. However, an operator has to aim at the vehicle and the driver at fault has to be apprehended shortly following his infraction.
Furthermore, inductive loops installed under the roadway also exist. However, the precision of the measurement is not optimum and the cost of installation of an inductive loop into the roadway is a significant intervention and very costly. Furthermore, these loops integrated into the roadway are subjected to large stresses, in particular by heavy trucks, reducing their lifetimes.
The documents EP 0 629 861, U.S. Pat. No. 6,208,268 and U.S. Pat. No. 5,954,299 describe devices for measuring the speed of terrestrial vehicles, comprising two sensors disposed one behind the other at a predetermined distance along the road at the side of the latter and comprising a measurement unit connected to the sensors which determines the phase-shift or the time-shift of the electrical output signals from the sensors and calculates, from these, with the distance of the sensors, the speed of the vehicles.
The sensors are magnetometers which detect the deformation of the Earth's magnetic field by the vehicle (ferromagnetic overall).
Although the price of these sensors is affordable, these installations suffer from a lack of precision, such that their use, notably in urban areas requiring a more precise speed measurement, is not appropriate.
The document 102 0 658 describes an analogous installation with an overhead gantry on which are installed for each lane two magnetometers, one behind the other in the direction of flow of the vehicles.
The document U.S. Pat. No. 3,860,869 describes a device for measuring the speed of a terrestrial vehicle with a single magnetometer which detects the speed along a rectilinear path.
However, this solution described with military vehicles is not adapted to a situation for monitoring traffic where the vehicles can closely follow one another.